FIG. 1a shows a section of an Intermediate Pressure (IP) compressor 10 from a three-shaft gas turbine. As shown, air may be bled from a mid stage of the IP compressor via a duct 12 to pressurise the fan disk and/or front 14 of the IP compressor 10, e.g. for sealing purposes. However, the high-pressure bleed air may leak into the mainstream 15 through a front seal 16, which is marked with a circle in FIG. 1a and shown in greater detail in FIG. 1b. 
It is known to provide seals between moving and stationary components, e.g. a rotor disk 22 and seal carrier 23, and typically such seals comprise labyrinth seals. The front seal 16 shown in FIGS. 1a and 1b may comprise such a labyrinth seal. Generally, there are two types of labyrinth seal, the first being a “straight through” type with a succession of upstanding edged fins extending across the leakage gap. The second type of labyrinth seal, as shown in FIG. 1b, may comprise a “stepped” labyrinth seal in which there are again a succession of upstanding edged fins 18, but the opposed surface 20 is stepped to convolute the flow path. A leakage through the gaps between the upstanding edged fins and the opposed surfaces may therefore be further constricted. Examples of labyrinth seal are disclosed in the following documents: US2008124215, US2009067997, U.S. Pat. Nos. 5,029,876, 3,572,728, 3,940,153 and 7,445,213.
Typically, the edged fins of a labyrinth seal are formed from solid metal with sharp machined edges to maximise the constriction of flow through the leakage gap. It will be understood that this leakage is due to a pressure differential across a rotary component, which may be a stage of a compressor or turbine in an engine. This pressure differential drives the blades or vanes of the turbine (or vice versa in the case of a compressor). Therefore any leakage about the edges of these blades or vanes through the leakage gaps reduces the efficiency as this pressurised working fluid provides no work (or in the case of a compressor requires further work) and may present detrimental mixing losses.
The effectiveness of a labyrinth seal is subject to a number of factors. These factors include manufacturing constraints, in service conditions and geometrical limitations. Typically, the clearance between the upstanding fin and its opposed surface is a significant factor with regard to the specification of an appropriate seal. This clearance dimension should be as small as possible within the housing but without rotating part clashes or touching during normal operation.
Multiple constrictions in series may reduce the leakage mass flow by reducing the pressure drop across each constriction, hence reducing the leakage velocity through the clearance. The leakage flow is typically choked at the last fin. In previously-proposed front seal designs, as shown in FIG. 1b (and FIG. 3a), a high-speed jet 21 at the exit of the last fin hits the rotor blade disk 22, and stays attached to the disk, thereby increasing windage losses. In addition, the leakage enters the mainstream flow 15 as a cross-flow with a high radial velocity and radial angle. This increases mixing losses and aerodynamic spoiling at the IP compressor inlet. Furthermore, the leakage air is at a higher temperature than the mainstream, and thus has detrimental effect on the efficiency.
The present disclosure therefore seeks to address these issues.